Facilitation and feedback in the hypothalamo-pituitary-adrenal axis during food restriction in rats.

After 4 weeks of food restriction to 50% of ad libitum intake in rats, plasma corticosterone levels were increased, without any change in adrenal weight, and with no evidence of sympathetic nervous system activation (as measured by Tyrosine Hydroxylase and Phenylethanolamine N-Methyl Transferase activities in the adrenal gland). Plasma corticosterone levels were normalized wih the addition of 35% of the calories as sugar. The adrenocortical axis activity was therefore investigated in more detail (nycthemeral cycle of corticosterone levels, ACTH and corticosterone response to a CRF challenge) in ad libitum fed rats and in animals fed 85% or 50% of the intake of the control animals, just before switching the lights off in order to maintain the diurnal rhythm of food intake. Food restriction to 85% did not change mean plasma corticosterone levels but sharpened the peak of corticosterone measured in the evening, indicating that the adrenocortical axis is more sensitive to the endogenous signals responsible for its diurnal cycle of activity. Indeed, the ACTH response to CRF was also increased. A 50% food restriction regimen increased mean corticosterone levels and attenuated the day/night difference, with high corticosterone levels maintained throughout the day. However, the ACTH response to CRF was not different from that measured in ad libitum fed rats, but the corticosterone response was lower, confirming that the adrenal gland is hyposensitive to ACTH. The results are discussed in terms of the balance between inhibiting/activating mechanisms and endocrine/neural influences at each level of the hypothalamo-hypophyso-adrenal axis, depending on the level of food deprivation.